In the medical field, active progress has been made in research of optical imaging technology that forms an image of information from the interior of a living organism, which is obtained on the basis of incident light when light is irradiated onto the living organism from a light source such as a laser. One optical imaging technology of this kind is Photoacoustic Tomography (PAT). In photoacoustic tomography, pulsed light generated from a light source is irradiated onto a living organism, an acoustic wave is generated from the living tissue which has absorbed the energy of the pulsed light that has propagated or diffused inside the organism, and this acoustic wave (typically, an ultrasonic wave) is detected. In other words, using the difference in absorptivity of the light energy in an examination site, such as a tumor, and other tissue, the elastic wave generated when the examination site momentarily expands upon absorbing irradiated light energy is received with an acoustic probe (also called a probe or transducer). By analyzing this detection signal, it is possible to obtain an image which is directly proportional to the initial pressure distribution or the light absorption energy density distribution (the product of the absorption coefficient distribution and the light amount distribution) (NPL 1).
Furthermore, by performing measurement using light of various wavelengths, this image information can be used for quantitative measurement of specific properties of the organism, such as the total hemoglobin density or blood oxygen saturation, or the like. In recent years, photoacoustic tomography of this kind has been use to make active progress in preclinical research for creating images of blood vessels in small animals, and clinical research which applies this principle to the diagnosis of breast cancer, prostate cancer, carotid artery plaque, and so on.